Bioconversion refers to the conversion of organic materials (generally wastes) into useful byproducts by processes (such as fermentation) involving living organisms. Bioconversion is also generically known as anaerobic digestion (AD), which is a process commonly utilized for pollution control in municipal sewage treatment and livestock waste handling. Bioconversion technology can also be applied to other organic "waste" streams, which might not otherwise be "treated" or "treatable". Some examples of these types of waste are: pre- and post-consumer food waste, "green" waste (cut grass, shrub and tree trimmings, etc.), waste paper (magazines and junk mail, mixed residential, etc.), FOG wastes (fats, oils, and grease), and "high-strength" wastewaters.
In anaerobic digesters, digestion rates are reduced due to the lack of enzymes necessary for complete digestion. This lack of enzymes can be attributed to: (1) poor growth of the bacteria which produce these enzymes; (2) the lack of access of the appropriate and acclimated bacteria to the feedstock; (3) feedback inhibition of enzyme production due to byproducts in intimate contact with the bacterial cells; and (4) inhibition of enzyme production can be due to high concentrations of byproduct intermediates in the fermentation fluid. Low rates of digestion can also be due to fresh feedstock slurry displacing the slurry of settled aggregates of active enzyme-producing bacteria, and those bacteria attached to the digested solids.
Low digestion rates can be overcome by recycling large volumes of effluents high in suspended acclimated bacterial biomass with the new input, or by recirculation of effluent, five to six times the volume of fresh input. However, this approach increases the total hydraulic retention time, and thereby, increases the working volume for the total digestion system. This in turn, requires additional vessel construction to accommodate the added volume necessary for recycle, and adds additional capital costs for that construction and operating costs to accommodate the energy and pumps used for pumping said recirculation volume.
A more efficient means of digestion can be attained by the incorporation of bacterial immobilization. The prior art has shown various approaches to the use of immobilizing matrices made of a wide range of materials. Prior art indicates that rocks, zeolites, clay particles, uncoated ceramic tile, and shaved ceramic have been used in packed bed reactors for digestion of soluble solids. Rigidly formed plastic media has been used in packed bed and anaerobic filters of soluble wastes. Heat bonded unwoven and woven fibers, and plastic films have been used in low solids digestion applications. Polyurethane foam pieces have been used in fluidized bed systems for cellular attachment to improve soluble nutrient conversion. Alginate gel beads with immobilized attached organisms within the interstices of the gel have been used in fluidized bed reactors for cellular growth and byproduct with soluble nutrients as the substrate.
The natural characteristics of microorganisms and cultured cells and how those characteristics affect immobilization have not been fully understood. Minerals, ceramics, clays and plastics with positive charged surfaces provide immobilizing surfaces for microorganisms due to attraction to negative ions which exist at the cellular surface. The known types of immobilizing surfaces, in an anaerobic filter, or packed bed reactor, can blind off or plug up with continued organism growth and subsequent attachment due to these charged effects of the individual cells. This reduces diffusion across the cell membrane, thus reducing enzyme access to substrate feedstocks and further encourages the development of aggregates of colonies filling microscopic pores and interstitial spaces of pore-related matrices. Mineral elements and metals in solution bond to the surface of these cell membranes. This adds to the overgrowth of cell populations on previous layers, resulting in blinding off of surfaces where active biomass grows.
Polyvinyl and, in particular, polyester of certain industrial types have a surface charge which is the result of polymer linkages generated in their respective manufacturing processes.
These plastic materials become irreversibly loaded with immobilized cells and with the soluble minerals extracted by immobilized cells from the fermentation liquor and are rapidly reduced in enzyme activity and cellular diffusion due to this charge effect.
Prior art digesters have employed various types of structures in an attempt to achieve high process flow and digestion rates. For example, U.S. Patent to Raymond, U.S. Pat. No. 4,274,838 to Dale, U.S. Pat. No. 4,334,997 to Peterson, U.S. Pat. No.4,551,243 to Martin, U.S. Pat. No. 4,885,094 to Srinivasan, U.S. Pat. No. 5,096,579 to Jordan et al., U.S. Pat. No. 5,403,742 to Freeman, and U.S. Pat. No. 5,525,229 to Shih disclose horizontal fluid flow through a trough, labyrinthine, vaned, or bed-like structure. Others, such as U.S. Pat. No. 4,318,993 to Ghosh and U.S. Pat. No. 5,500,123 to Srivastava show multi-stage component vessels. U.S. Pat. No. 5,413,713 to Day et al. and U.S. Pat. No. 5,618,412 to Herding show the use of a columnar or vertical vessel with trickle-down through horizontal bed layers. U.S. Pat. No. 5,499,770 to McCullough discloses the use of a vessel with a vortex fluid flow.
While certain advantages are obtained in the use of one or another type of structure, the digesters of the prior art have not been found to achieve an optimal combination of desired digester properties, including high surface area to liquid volume ratio, high digestion rates, good immobilization of biomass, and efficient culturing and use of microorganisms.